This invention relates primarily to metallic coatings and coated articles and, more particularly, to metallic coatings applied to metal articles for high temperature use.
As modern power generation apparatus, such as the gas turbine engine, has evolved, the environmental operating temperatures in its hotter sections have increased. Although metallurgists have developed improved alloys from which metallic components can be made, some are subject to surface deterioration such as through oxidation or hot corrosion, to a degree greater than that which is desirable. Therefore, concurrently with the evolution of such apparatus has been the development of high temperature operating surface treatments and coatings.
From the literature, it can be seen that a large number of such coatings involve the use of aluminum as an important ingredient in the coating. Earlier methods involved applying aluminum metal to the surface directly such as through dipping in molten aluminum or spraying molten aluminum onto the surface of an article. Such methods resulted in an increase in article dimensions. Therefore, in order to retain the critical dimensions of an article such as for use in gas turbines, the pack diffusion process was developed. One example of such a pack process is represented by U.S. Pat. No. 3,667,985 -- Levine et al. issued June 6, 1972. Vapor deposition of high temperature coatings, including aluminum as an important ingredient, is shown in one form in U.S. Pat. No. 3,528,861 -- Elam et al. issued Sept. 15, 1970. Another method for vapor depositing coatings on a substrate is shown in U.S. Pat. No. 3,560,252 -- Kennedy issued Feb. 2, 1971. The disclosure of each of these patents is incorporated herein by reference.
Although a number of methods, compositions and mixtures have been developed for the purpose of inhibiting or retarding surface deterioration of articles exposed to the environment at elevated temperatures, each has its limitation in respect to the length of time it can afford protection.